When a plurality of cells using the same frequency channel are situated in proximity to each other, due to the interference of each other's radio waves, the communication quality of a wireless communication system is markedly degraded. It is for this reason that the frequency reuse distance is usually calculated with consideration given to the power attenuation due to a propagation path loss, and base stations using the same frequency channel are disposed so as to be geographically away from each other.
With the recent spread of various wireless communications and conversion to broadband wireless communication, the communication capacity of the wireless communication system is expected to increase. To meet that requirement, there is a method of increasing the number of base stations. However, the frequency band suitable for wireless communication, typified by the microwave band, has already become scarce, and it is not easy to increase the number of base stations due to the above-described interference associated with the same frequency channel.
As a technique to hold a plurality of users in the same frequency channel, there are spatial multiplexing techniques such as space division multiple access (SDMA) and multiuser multi input multi output (MIMO) (Documents 1 and 2). In SDMA, user multiplexing for the same frequency channel is realized by performing space separation of the wireless resource on a user-by-user basis based on direction of arrival estimation technology or the like. In multiuser MIMO, the signals spatially multiplexed on the same frequency channel are demultiplexed by multiuser detection by using different propagation paths formed between a plurality of transmitting and receiving antennas.
As a technique different from the spatial multiplexing technique, code division multiple access (CDMA) is known (Document 3). In CDMA, the signals of a plurality of users or a plurality of cells can be separated by using spread sequences with a low correlation value, it can be said to be a wireless communication system that tolerates interference associated with the same frequency channel. Therefore, when a new base station is added, the addition can be realized by relatively flexible base station allocation planning from the point of view of interference.    Document 1: Winters, J. H., “Smart antennas for wireless systems”, IEEE Personal Communications, Vol. 5, No. 1, pp. 23-27, February 1998    Document 2: Kai-Kit Wong, Murch, R. D., Letaief, K. B., “Performance enhancement of multiuser MIMO wireless communication systems”, IEEE Transaction on Communications, Vol. 50, No. 12, pp. 1960-1970, December 2002    Document 3: Hui, J., “Throughput Analysis for Code Division Multiple Accessing of the Spread Spectrum Channel”, IEEE Journal on selected areas in Communications, Vol. SAC-2, No. 4, pp. 482-486, July 1984